Conventional ethylene oligomerisation technologies produce a range of α-olefins following either a Schulz-Flory or Poisson product distribution. By definition, these mathematical distributions limit the mass % of the tetramer that can be formed and make a distribution of products. In this regard, it is known from prior art (U.S. Pat. No. 6,184,428) that a nickel catalyst comprising a chelating ligand, preferably 2-diphenyl phosphino benzoic acid (DPPBA), a nickel precursor, preferably NiCl2.6H2O, and a catalyst activator, preferably sodium tetraphenylborate, catalyse the oligomerisation of ethylene to yield a mixture of linear olefins containing 1-octene. The selectivity towards linear C8-olefins is claimed to be 19%. Similarly the Shell Higher Olefins Process (SHOP process, U.S. Pat. Nos. 3,676,523 and 3,635,937) using a similar catalyst system is reported to typically yield 11 mass % 1-octene in its product mixture (Chem Systems PERP reports 90-1, 93-6 and 94/95S12).
Ziegler-type technologies based on trialkylaluminium catalysts, independently developed by Gulf Oil Chemicals Company (Chevron-Phillips, e.g. DE patent 1,443,927) and Ethyl Corporation (BP-Amoco, e.g. U.S. Pat. No. 3,906,053), are also commercially used to oligomerise ethylene to mixtures of olefins that reportedly contain 13-25 mass % 1-octene (Chem Systems PERP reports 90-1, 93-6, and 94/95S12).
The prior art also teaches that chromium-based catalysts containing heteroatomic ligands with both phosphorus and nitrogen heteroatoms selectively catalyse the trimerisation of ethylene to 1-hexene. Examples of such heteroatomic ligands for ethylene trimerisation include bis(2-diethylphosphino-ethyl)amine (WO 03/053891) as well as (o-methoxyphenyl)2PN(methyl)P(o-methoxyphenyl)2 (WO 02/04119). Both these catalyst systems and processes are very specific for the production of 1-hexene.
Processes wherein transition metals and heteroatomic ligands are combined to form catalysts for trimerisation, tetramerisation, oligomerisation and polymerisation of olefinic compounds have also been described in WO 03/053890, WO 03/053891, WO 04/056479, WO 04/056477, WO 04/056478, WO 04/056480 and South African provisional patent application number 2004/3805.
It has now been found that the oligomerisation of olefinic compounds by catalysts containing a transition metal and a ligand can be enhanced by carrying out the reaction in an aliphatic medium. In processes according to the present invention aliphatic solvents such as cyclohexane provided more active reaction systems compared to aromatic solvents, which can reduce catalyst consumption significantly. In at least some cases it has also been found that aliphatic solvents such as cyclohexane had the effect that the catalysts were less prone to deactivation at temperatures above 45° C. compared to reactions where aromatic solvents were used at the same temperatures. Certain activators such as MMAO had also shown improved activity and stability of the catalyst when used in aliphatic solvents such as cyclohexane. Increased pressures also enhanced the reaction.